A patient may generate unexpected movement during surgical proceedings or may be required to relax his/her muscles to a certain degree. Therefore, a drug such as a muscle relaxant needs to be injected into the patient. After injection of the drug, monitoring muscle relaxation is usually performed on the patient to evaluate the degree of muscle relaxation. The degree of muscle relaxation can be detected by measuring the contraction strength of the adductor brevis of the patient's thumb. This is because the contraction strength reflects the degree of muscle relaxation. The strength is proportional to the acceleration according to Newton's law. Accordingly, the contraction strength of the adductor brevis of the thumb can be indirectly obtained by measuring the acceleration generated by movement of the thumb.
The level of muscle relaxation is usually measured by monitoring acceleration values generated by moving the thumb to output acceleration sampled values, and deriving angular velocity values by the acceleration values with some motion prediction. However, there exists bias between the motion prediction and actual movement of the thumb, resulting in inaccurate measurement of muscle relaxation. For example, suppose that the thumb moves in a circle around a fixed point (e.g., the base of the thumb), then a linear velocity can be acquired by the time integral of acceleration, and an angular velocity can be computed by dividing the linear velocity by an estimated length of the thumb. Thus all information required by measuring muscle relaxation can be obtained. Obviously, the actual movement of the thumb is not entirely a circular motion; therefore, such motion prediction may introduce an error in the measurement result.